Binary information is recorded, deleted and deciphered in memories using change of physical properties in electrical, magnetic, optical characteristics, and the like, of materials. Among such memories, a PRAM (Phase change Random Access Memory) to be in the limelight as the upcoming nonvolatile memory is a memory that information is recorded, deleted and deciphered using change in optical and electrical characteristics of materials according to the phase change. An optical memory utilizes reflectance to be different on changing a material state with a laser, while a phase change memory utilizes change of electrical conductance (specific resistance) on changing it with electrical signals. That is, when a material is in an amorphous phase, it has very high specific resistance. When it is in a crystalline phase that atoms are regularly arranged, it has low specific resistance. Then, binary information may be stored, deleted and deciphered using a different electrical conductance according to such a state.
When upper and bottom metal electrodes are linked to a power circuit, following applying voltage, current is carried through a part that a phase change material is in contact with the electrodes, wherein Joule's heat is generated at the contact areas according to Joule's law. As the electrical conductance (specific resistance) is changed on changing the material state by Joule's heat, a crystalline phase material and an amorphous material may be switched into an amorphous phase material and a crystalline phase material, respectively, using electrical signals. Since the resulting two phases are very stable, they are not erased even though power of the device is off, so that they may be used as nonvolatile memories.
In other words, recording of information (SET) refers to changing the material state from an amorphous phase (off state) to a crystalline phase (on state), for which long low current pulses are provided for an amorphous phase material (FIG. 1a). On the contrary, if short high current pulses are provided for a crystalline material being in on state to delete information (RESET), the crystalline phase is changed into the amorphous phase to be in off state (FIG. 1b).
Such a phase change memory has a simple structure compared with other nonvolatile memories and a benefit that an input-output speed is fast. It has been developed as a device having a thin film form based on GST (Ge, Sb, Te) being chalcogenides so far, and also researched for materials by means of doping elements such as Se and Bi. Recently, a variety of trials are carried out by realizing a PRAM or forming a thin film with nanowires prepared by high temperature processes such as CVD. However, in such processes, there are disadvantages that composition ratios in said elements are not easily controlled and they have complicated preparation processes and high production costs per unit.